The cooling power required to do this is the mechanical or electrical power required by the cooling devices, such as helium liquifiers, for example, needed to keep the superconductors at their operating tempeatures. The invention is particularly suited to machines in which the superconductors are cooled to a temperature at which their superconductivity is maintained by means of a cryogenic fluid such as helium passed through cooling circuits. After passing through the machine, this fluid, or part of it, may be used for cooling the current leads. If all the cooling fluid is used, its flow rate depends on the requirements of the machine, and cannot be modified to take account of the specific requirements of the current leads.